Typically, computer graphics operations with a pointing device are of two types; drawing operations and control operations. Drawing operations describe loci for the placement of visible marks on the image, analogous to drawing on a piece of paper by making a motion with a pen in contact with the surface. Control operations are used to initiate and guide the execution of computer functions leading to modification of the visible marks. Control operations are especially important to the performance of editing functions such as erasing, moving, or changing the visual qualities (e.g. line width) of visible marks in the image.
Underlying such an interactive computer-assisted graphical editing tool is means for maintaining information about the graphic elements of the image that may be selected and modified under user control. The various extant computer-assisted drawing tools may be classified along a spectrum defined by the granularity of the manipulable graphical elements. On the one hand, so-called paint programs manipulate the visual content of images at the level of individual pixels, each of whose lightness and/or color properties is modifiable independently from the other pixels. While paint style programs permit detailed refinement of an image, their inability to define and geometrically transform whole groups of pixels makes many operations extremely tedious to perform. On the other hand, structured-graphics editing programs operate at the level of larger abstract objects such as lines, circles, and polygons whose rendition on the imaging surface includes many pixels, all of which are modified in unison when objects are moved, deleted, or otherwise modified. The rationale for structured-graphics editing programs lies in an assumption that the sorts of image modifications that users will usually want to perform will be ones that preserve the geometric structures reflected in the abstract level objects.
The present invention offers a new point in the spectrum of computer assisted drawing tools, one that incorporates advantages from both the paint and structured-graphics extremes of graphical image editors.